The present invention relates generally to the art of forming and sintering ceramic powders and is particularly concerned with a method for sintering a uranium dioxide nuclear fuel body having a fugitive binder.
Various materials are used as nuclear fuels for nuclear reactors including ceramic compounds of uranium, plutonium and thorium with particularly preferred compounds being uranium oxide, plutonium oxide, thorium oxide and mixtures thereof. An especially preferred nuclear fuel for use in nuclear reactors is uranium dioxide.
Uranium dioxide is produced commercially as a fine, fairly porous powder which cannot be used directly as nuclear fuel. It is not a free-flowing powder but clumps and agglomerates, making it difficult to pack in reactor tubes to the desired density.
The specific composition of a given commercial uranium dioxide powder may also prevent it from being used directly as a nuclear fuel. Uranium dioxide is an exception to the law of definite proportions since "UO.sub.2 " actually denotes a single, stable phase that may vary in composition from UO.sub.1.7 to UO.sub.2.25. Because thermal conductivity decreases with increasing O/U ratios, uranium dioxide having as low an O/U ratio as possible is preferred. However, since uranium dioxide powder oxidizes easily in air and absorbs moisture readily, the O/U ratio of this powder is significantly in excess of that acceptable for fuel.
A number of methods have been used to make uranium dioxide powder suitable as a nuclear fuel. Presently, the most common method is to die press the powder into cylindrically-shaped green bodies of specific size without the assistance of fugitive binders since the complete removal of these binders and their decomposition products is difficult to achieve prior to sintering. The entrainment of binder residues is unacceptable in sintered nuclear fuels. Sintering atmospheres may range from about 1000.degree. C. to about 2400.degree. C. with the particular sintering temperature depending largely on the sintering atmosphere. For example, when wet hydrogen gas is used as the sintering atmosphere, its water vapor accelerates the sintering rate thereby allowing the use of correspondingly lower sintering temperatures such as a temperature of about 1700.degree. C. The sintering operation is designed to densify the bodies and bring them down to the desired O/U ratio or close to the desired O/U ratio.
Although uranium dioxide suitable as a nuclear fuel can have an O/U ratio ranging from 1.7 to 2.015, as a practical matter, a ratio of 2.00 and suitably as high as 2.015 has been used since it can be consistently produced in commercial sintering operations. In some instances, it may be desirable to maintain the O/U ratio of the uranium dioxide at a level higher than 2.00 at sintering temperature. For example, it may be more suitable under the particular manufacturing process to produce a nuclear fuel having an O/U ratio as high as 2.195, and then later treat the sintered product in a reducing atmosphere to obtain the desired O/U ratio.
One of the principal specifications for uranium dioxide sintered bodies to be used for a nuclear reactor is their density. The actual value may vary but in general uranium dioxide sintered bodies having densities of the order of 90% to 95% of theoretical density are specified and occasionally a density as low as 85% of theoretical is specified. Most pressed uranium dioxide powder, however, will sinter to final densities of about 96% to 98% of theoretical. Therefore, to obtain sintered bodies with lower densities the time and temperature must be carefully controlled to allow the shrinkage of the body to proceed only to the desired value. This is inherently more difficult than the use of a process which is allowed to go to completion. Specifically, small variations during sintering can result in large variations or no significant variations in the sintered body of compacted powder depending on a number of factors such as the powder chemistry, particle size and agglomeration. Generally, however, a change in sintering time such as, for example an hour or two, does not significantly change the density of the final sintered product. Also, when sintered bodies having the desired low density have been attained by carefully controlling sintering time and temperature, it has been found that these sintered bodies, when placed in the reactor, frequently undergo additional sintering within the reactor thereby interfering with proper reactor operation.
A number of techniques have been used in the past to reduce the density of the sintered body other than varying time and temperature. For example, one technique has been to press the uranium dioxide powder, break it up and repress it. The problem with this technique is that the resulting sintered body has large interconnecting pores throughout the body which extend out to the surface resulting in a large exposed surface area which can absorb into the body significant amounts of gases, and in particular water in the form of water vapor. During reactor operation these gases are liberated providing a possible source of corrosion for the fuel cladding. Another method involves adding a plastic of selected particle size to the uranium dioxide powder. The admixed powder is then pressed and sintered, however the decomposition of the plastic during sintering usually results in carbon residues which contaminate the nuclear fuel.
In U.S. patent application Ser. No. 437,837 filed Jan. 30, 1974 in the name of Kenneth W. Lay (and now abandoned) and assigned to the same assignee as the present invention, there is disclosed a process for controlling the end-point density of a sintered uranium dioxide nuclear fuel body and the resulting product. Uranium dioxide powder having a size ranging up to 10 microns is admixed with a precursor to uranium dioxide, such as ammonium diurante, having an average agglomerated particle size ranging from about 20 microns to 1 millimeter and the mixture is formed into a pressed compact or green body. The body of the precursor and the uranium dioxide has a density lower than that of the uranium dioxide powder and the precursor is used in an amount which results in discrete low density regions in the green body which range from about 5% to 25% by volume of the green body. The green body is sintered to decompose the precursor and produce a sintered body having discrete low density porous regions which reduce the end-point density of the sintered body by at least 2%. The sintered body has an end-point density ranging from 85% to 95% of theoretical.
In copending U.S. patent application Ser. No. 598,839, filed July 24, 1975 (and now abandoned) and assigned to the same assignee as the present invention there is disclosed a process for controlling the final or end-point density of a sintered uranium dioxide nuclear fuel body by adding ammonium oxalate to a nuclear fuel material such as uranium dioxide before pressing into a green body. This addition results in discrete low density porous regions in the sintered body which correspond to the ammonium oxalate particles. The end-point density of the sintered body is, therefore, a function of the amount of ammonium oxalate added.
As previously mentioned, conventional organic or plastic binders are unsuitable for use in powder fabrication since they tend to contaminate the interior of the sintered body with impurities such as hydrides. These binders are normally converted to gases during the sintering step and these gases must be removed, requiring special apparatus or procedures. In addition, upon decomposition, these prior art binder materials often leave deposits of organic materials in the equipment utilized to sinter the article, thereby complicating the maintenance procedures for the equipment.
In the sintering process, it is desirable to develop strong diffusion bonds between the individual particles without significantly reducing the interconnecting porosity of the body. The use of organic binders along with normal compacting pressures and sintering temperatures inhibits the formation of these strong bonds. The higher compacting pressures and sintering temperatures required to develop such bonds sharply reduce the desired porosity.
There is a particular need, therefore, in the art of preparing sintered bodies for nuclear reactors by powder ceramic techniques for a binder which will impart an adequate degree of green strength without contaminating the interior of such bodies and which will permit, through sintering, the formation of strong bonds between particles without deleteriously affecting the porosity.